As the size of integrated circuit features continues to decrease, the accuracy of measurements becomes more important. Due to greater emphasis on precision rather than accuracy, many of the measurements made in semiconductor fabs are not traceable to the SI (Syst me International d Unites or International System of Units) unit of length. However as feature sizes decrease and the use of lithography models becomes more prevalent, the need for accuracy cannot be overemphasized. In response, the National Institute of Standards and Technology (NIST) in conjunction with SEMATECH has developed a reference measurement system (RMS) that can be used to provide accurate measurements for inline metrology tools. The RMS is a critical dimension atomic force microscope (CD-AFM) with traceability to the SI meter. In this paper we present a set of strategies for achieving accuracy for different types of measurands within an RMS and examine some of important factors when selecting reference instruments. We also present results of a recent evaluation of linewidth and height using two CD-AFMs and a calibrated AFM with displacement interferometry in all three axes. We further look at the stability of tips such as carbon nanotubes.